Change in Solvation of Poly(N,N-diethylacrylamide) during Phase Transition in Aqueous Solutions As Observed by IR Spectroscopy

Phase transitions of poly( N, N -dimethylacrylamide) (PdEA) in water and methanol/water mixture were investigated by Fourier transform infrared (FTIR) spectroscopy. IR spectra of the solutions measured as a function of temperature exhibited critical changes upon the phase transition. The amide I band of PdEA consists of three components centered at 1638, 1619, and 1599 cm -1, which can be assigned to free carbonyl groups and those connected to one and two water molecules through hydrogen bonds, respectively. The relative areas of the 1619 and 1599 cm -1 components decrease and that of the 1638 cm -1 component increases above the phase transition temperature ( T p ), but the change is relatively small and most of the amide C O groups remain hydrated even above T p . The C−H stretching bands shift toward lower wavenumbers that are close to those measured in the neat solid state, indicating that the alkyl groups are almost fully dehydrated above T p . In the methanol/water mixture, T p is almost independent of methanol concentration up to ca. 20% and increases steeply above ca. 30%. The heat of transition (Δ H ) decreased linearly with an increase of methanol concentration up to 30% and is nearly zero above 40%. The areas of the 1599 and 1638 cm -1 bands decrease with an increase in methanol concentration with a slight change in the 1619 cm -1 band. The result suggests that methanol replaces parts of the C O···H−O−H hydrogen bonds to form a C O···H−O−CH 3 hydrogen bond, and its bulky methyl group prevents the carbonyl group to form the second hydrogen bond that is responsible for the 1599 cm -1 band.